Semiconductors are widely used in integrated circuits for electronic applications, including radios and televisions. Such integrated circuits typically use multiple transistors fabricated in single crystal silicon. Many integrated circuits now contain multiple levels of metallization for interconnections. As geometries shrink and functional density increases, it becomes imperative to reduce the RC time constant within multi-level metallization systems.
Although the dielectric typically used in the past to isolate metal lines from each other was silicon dioxide, recent trends have been towards using materials with low-dielectric constants in order to reduce the RC time constant. Many low-dielectric insulators are either pure polymers (e.g. parylene, BCB, teflon, polyimide) or organic spin-on glass (OSOG, e.g. silsequioxane and siloxane glass). The oxidation resistance of these low-dielectric materials is generally poorer than the oxidation resistance of silicon dioxide.
Conventional ash processes are available for high-aspect-ratio contacts and vias; however, such processes are damaging to dielectric materials, especially low-dielectric materials. Oxygen plasma (ash) is commonly used to remove photoresist from wafer surfaces. The conventional ash process is performed in barrel and down-stream reactors, and is carried out at a high temperature (approximately 250.degree. C.) for a long period of time to assure that all photoresist is completely cleared. Unfortunately, high temperature ash processes not only clear photoresist from the surface of the wafer, but also remove polymer contents from low-dielectric materials, resulting in problems such as shrinking, cracking, moisture absorption, via poisoning, undercutting, and general degradation of the dielectric property of low-dielectric materials. Via integration has presented a problem in the use of low-dielectric materials.
Thus, the use of low-dielectric constant materials in the semiconductor industry has resulted in the need for an ash process that does not damage polymeric dielectric materials while removing the photoresist.